Zircon compatible glasses for down draw

ABSTRACT

A glass that is down-drawable and ion exchangeable. The glass has a temperature T 35kp  at which the viscosity is 35 kilopoise. T 35kp  is less than the breakdown temperature T breakdown  of zircon.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/235,762 filed on Aug. 21, 2009.

BACKGROUND

Ion-exchangeable glasses can be manufactured in several forms by a variety of processes. In particular, such glasses can be drawn into thin sheets via either slot-draw or fusion-draw processes.

Current fusion-draw designs require zircon refractory lining and hardware for isopipes. Most ion-exchangeable glasses react with the zircon, breaking it down into silica, which dissolves in the glass, and zirconia, which forms solid inclusions that are entrained by flow into the molten glass and ends up in the final product. The attack of zircon by the molten glass continues over time and the level or concentration of zirconia inclusions in the glass increases. Because these inclusions are concentrated on the fusion line, they are also located at the point of maximum central tension after ion exchange, potentially compromising the strength of the ion-exchanged glass part. Moreover, in applications, such as in hand-held electronic devices such as PDAs, mobile phones, etc., in which small pixels of light are to be directed through the glass sheet, the high refractive index of the zirconia inclusions can block pixels. In very thin (e.g., ≦1 mm thick) glass sheets, the zirconia inclusions are visible to the human eye, and constitute cosmetic defects that, in some instances, result in rejection of the part.

SUMMARY

What is provided and described herein is a glass that is down-drawable and ion exchangeable. The glass has a temperature T^(35kp) at which the viscosity is 35 kilopoise. T^(35kp) is less than the breakdown temperature T^(breakdown) of zircon.

Accordingly, one aspect of the disclosure is to provide a glass. The glass comprises SiO₂ and Na₂O and has a temperature T^(35kp) at which the glass has a viscosity of 35 kpoise, wherein the temperature T^(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂ is greater than T^(35kp).

A second aspect of the disclosure is to provide a glass. The glass comprises SiO₂ and Na₂O and having a temperature T^(35kp) at which the glass has a viscosity of 35 kpoise, wherein SiO₂+B₂O₃≧66 mol % and Na₂O≧9 mol %, and wherein the temperature T^(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂ is greater than T^(35kp).

A third aspect of the disclosure is a fusion-drawn glass sheet that is substantially free of fusion line zirconia defects.

These and other aspects, advantages, and salient features will become apparent from the following detailed description, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of measured vs. predicted zircon breakdown temperature T^(breakdown);

FIG. 2 is a plot of measured values vs. predicted 35 kp temperatures T^(35kp);

FIG. 3 is a backscattered electron micrograph of a mixture of zircon and a glass composition (composition 5, Table 1) following heat treatment at 1175° C. for 114 hours; and

FIG. 4 is a backscattered electron micrograph of a mixture of zircon and a glass composition (composition 11, Table 1) following heat treatment at 1175° C. for 114 hours.

DETAILED DESCRIPTION

In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that, unless otherwise specified, terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as comprising at least one of a group of elements and combinations thereof, it is understood that the group may comprise, consist essentially of, or consist of any number of those elements recited, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a group of elements or combinations thereof, it is understood that the group may consist of any number of those elements recited, either individually or in combination with each other. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range.

Referring to the drawings in general, it will be understood that the illustrations are for the purpose of describing particular embodiments and are not intended to limit the disclosure or appended claims thereto. The drawings are not necessarily to scale, and certain features and certain views of the drawings may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

If the temperature at which the zircon in the isopipe breaks down to form zirconia and silica (also referred to herein as the “breakdown temperature” or “T^(breakdown)”) were higher than any temperature seen on the isopipe, the problem of zirconia inclusions in fusion-drawn glass (also referred to as “fusion line zirconia”) would not occur. In this instance, the temperatures used to form the glass over the isopipe would be too low to create zirconia, and no such defect could form in the glass.

Because fusion is essentially an isoviscous process, the highest temperature seen by the glass corresponds to a particular viscosity of the glass. In those standard fusion-draw operations known in the art, this viscosity is about 35,000 poise (“35 kpoise” or “35 kp”), although the viscosity can be as low as about 16,000 poise during brief periods when an isopipe is being initially wet by glass. We define the difference between the breakdown temperature and the temperature corresponding to 35,000 poise viscosity as the breakdown margin T^(margin), where: T ^(margin) =T ^(breakdown) −T ^(35kp),  (1) where T^(35kp) is the temperature at which the viscosity of the glass is 35,000 poise.

When the breakdown margin T^(margin) is negative, zircon will breakdown to form zirconia defects at some location on the isopipe. When T^(margin) is zero, it is still possible that temperature excursions could cause zircon breakdown to occur. It is therefore desirable not only to make the breakdown margin positive, but to maximize T^(margin) as much as possible while being consistent with all the other attributes that must be maintained in the final glass product.

To understand the relationship between breakdown temperature and 35 kp temperature, it is helpful to consider the reaction to form zirconia from zircon. The reaction is written thus: ZrSiO₄(xtal)→ZrO²(xtal)+SiO²(liq),  (2) where ZrSiO₄(xtal) and ZrO₂(xtal) are crystalline zircon and zirconia, respectively, and SiO₂(liq) is liquid silica, which dissolves into the glass, produced by reaction (2).

The breakdown of zircon can be prevented by driving reaction (2) to the left. To do so, the activity (i.e., the concentration) of at least one of ZrO₂ or SiO₂ in the glass is increased. To accomplish this via ZrO₂ addition, the concentration of ZrO₂ would have to be increased until zircon became the liquidus phase. This, however, would risk creating a new, unwanted defect, secondary zircon. The remaining option is to increase the activity/concentration of SiO₂ in the glass. As SiO₂ increases at the expense of other glass components, however, the glass viscosity increases as well. Thus, the rate at which breakdown temperature increases must be balanced against the rate at which the 35 kp temperature T^(35kp) increases.

In one aspect, a glass composition having a breakdown temperature T^(breakdown) that is higher than the 35 kp temperature T^(35kp)—i.e., T^(breakdown)>T^(35kp)—is provided. The breakdown temperature is preferably higher than any viscosity that might be obtained—however briefly—during normal operation of a fusion-draw process. The composition is, in one embodiment, meltable in a conventional electric-boost, zirconia or alumina-refractory tank; i.e., the melt temperature T^(350p) (i.e., the temperature corresponding to a viscosity of about 350 poise) of the glass is less than about 1650° C.

In one embodiment, the glass described herein, in addition to being down-drawable and compatible with fusion-draw processes, is ion-exchangeable so as to produce a compressive layer on the glass surface of at least 20 μm in depth and having a maximum compressive stress of at least 350 MPa. In other embodiments, the glass is ion-exchangeable so as to produce a central tension within the glass of at least 10 MPa. The glass comprises SiO₂ and Na₂O, wherein SiO₂+B₂O₃≧66 mol %, and Na₂O≧9 mol %. The glass, in some embodiments, further comprises at least one of B₂O₃, K₂O, MgO, and CaO. In a particular embodiment, the glass comprises: 61 mol %≦SiO₂≦75 mol %; 7 mol %≦Al₂O₃≦15 mol %; 0 mol %≦B₂O₃≦12 mol %; 9 mol %≦Na₂O≦21 mol %; 0 mol %≦K₂O≦4 mol %; 0 mol %≦MgO≦7 mol %; and 0 mol %≦CaO≦3 mol %. The glass, in some embodiments, further comprises at least one fining agent such as, but not limited to, a halogen or a multivalent fining agent such as As₂O₃, Sb₂O₃, SnO₂, and Fe₂O₃. In a particular embodiment, the total concentration of As₂O₃ and Sb₂O₃, if either or both are present, is less than 500 ppm by weight.

The breakdown margin T^(margin) of the glass is given by the expression T ^(margin)(° C.)=610.6−41.0[Al₂O₃]+9.9[B₂O₃]−3.5[Na₂O]−20.2[K₂O]−25.6[MgO]+34.2[CaO],  (3) where the concentrations [Al₂O₃], [B₂O₃], [Na₂O], [K₂O], [MgO], and [CaO] are expressed in mol %. The coefficients in equation (3) have been further refined to yield the expression T ^(margin)(° C.)=446.6−50.2[Al₂O₃]+22.6[B₂O₃]−4.4[Na₂O]−3.9[K₂O]−1.2[MgO+CaO].  (3a) The SiO₂ concentration is equations (3) and (3a) is substantially accounted for by difference from the other oxides and is therefore not included in this expression. When the condition in equation (3) is satisfied, the breakdown temperature for zircon will be greater than the 35 kp temperature of the glass (i.e., T^(breakdown)>T^(35kp)), and fusion-line zirconia defects can thus be avoided when the glass is formed over a zircon isopipe in the fusion process.

As can be seen from the regression coefficients in equation (3), the only oxide that contributes positively to breakdown margin T^(margin) is B₂O₃. In order to avoid excessively low breakdown temperatures, lower SiO₂ concentrations require higher B₂O₃ concentrations, since SiO₂ is the strongest contributor to high breakdown temperatures. The linkage between B₂O₃ and SiO₂ results in a more fundamental constraint on glass composition, namely, SiO₂+B₂O₃≧66 mol %.

Na₂O is the essential oxide component for facilitating ion exchange, as it is removed from the glass and replaced by larger monovalent cations such as, for example, K₂O during conventional ion exchange in a molten salt such as, for example, potassium nitrate (KNO₃). In order to obtain the minimum useful compressive stress, Na₂O≧9 mol %.

While it is desirable that the glasses described herein be compatible with existing melting processes, alternative means of melting or alternative batch materials that would enable melting harder glasses may also be used to prepare these glasses. To keep melt temperatures from becoming too high, fluxes (e.g., B₂O₃, alkali oxides, and alkaline earth oxides) can be kept at relatively high levels with respect to the basic glass formers Al₂O₃ and SiO₂. This can be understood from the fact that the 35 kp and 350 p temperatures “track” each other—i.e., a high 35 kp temperature also implies a high 350 p temperature. From regression equation (5) for 35 kp (shown below), high Al₂O₃ contents and, by inference, high SiO₂ contents lead to high 35 kp temperatures, and therefore to high melt temperatures. To avoid higher melt temperatures, it is desirable that B₂O₃+Na₂O+K₂O+MgO+CaO≧18 mol %. With the exception of MgO, all of the above components also contribute to low liquidus temperature, which ensures a liquidus viscosity that is high enough to be compatible with fusion-draw processes.

In one aspect, it is desirable that the liquidus temperature be as low as reasonably achievable to ensure a high liquidus viscosity. The relationship between liquidus temperature and composition is exceptionally complex, and no simple algorithm can be devised to account for it. However, liquidus temperature generally decreases as the concentration of alkali metal oxides in excess of aluminum (i.e., Na₂O+K₂O−Al₂O₃) increases, and as sodium is replaced by potassium, at least within the ranges indicated above. Likewise, liquidus temperature decreases steeply with increasing B₂O₃. Therefore, it is advantageous that B₂O₃+Na₂O+K₂O−Al₂O₃≧0 to ensure that the glass have suitably low liquidus temperature.

The glasses described herein may be fusion drawn into sheets that can be used as cover plates, windows, casings, screens, touch panels, or the like, for mobile electronic devices, such as telephones, entertainment devices, including games, music players and the like; information terminal (IT) devices, such as laptop computers; and analogous stationary versions of such devices.

Zircon breakdown temperature can be experimentally determined by one of several methods. In one method, known as the gradient strip test, a glass sample of interest is crushed and sieved to obtain a size fraction of generally less than 20 mesh. A strip of 1390 zircon refractory (Cohart) is placed in the bottom of a long, narrow platinum boat and the crushed glass is placed on top of the zircon refractory. The boat containing the refractory and glass is then loaded into a conventional gradient tube furnace that has been calibrated so that the temperature at each point in the furnace can be mapped to a specific place along the length of the boat. The gradient is set such that the low end of the furnace is at a temperature in a range from about 750° C. up to about 800° C., and the hot end is at a temperature in a range from about 1225° C. up to about 1300° C. The boat remains in the furnace for one week. After one week in the furnace, the boat is removed and the glass/refractory slab is sectioned along its length and inspected. Zirconia inclusions in the glass can be distinguished from zircon using polarized light microscopy, and further confirmation can be obtained via scanning electron microscopy. Because the furnace temperature is known along the length of the sample, the location at which zirconia first appears corresponds to a particular temperature. The estimated uncertainty in the temperature is about ±10° C.

Compositions of glasses evaluated by the gradient strip test method are shown in Table 1, and plotted vs. predicted breakdown temperature in FIG. 1. Compositions listed in Table 1 are expressed in mol %. In FIG. 1, the y-error bars denote the measurement uncertainty of 10° C., and the x-error bars denote the 2σ standard error—15° C.—of the regression equation. The compositions listed in Table 1 are expressed in mol % and, for most of the glasses listed, are the nominal compositions. The true compositions and nominal compositions should be close for most of the glasses listed in Table 1, but for the highest boron glasses the melting process itself may well have reduced the B₂O₃ content of the glasses. In several cases, the breakdown temperatures (“Breakdown T”) were higher than the temperature at the hot end of the gradient furnace. In these instances, it can only be established that the breakdown temperature T^(breakdown) was higher than the temperature of the hot end, and a unique temperature for T^(breakdown) cannot be assigned. For three of the samples (25, 26, 27) listed in Table 1, the high coefficient of thermal expansion of the glasses caused heavy crizzling or crazing of the glass at the glass-refractory interface. Consequently, it could only be determined that, for these samples, zirconia was present at up to about 1100° C., but the temperature (presumably lower) at which zircon became stable could not be determined.

Table 1 also lists predicted breakdown temperatures that were obtained by linear regression of the breakdown temperature against the mole fractions of all the major element oxides except SnO₂ (frequently present at a low level as a fining agent and SiO₂. SiO₂ was left out of the regression because its concentration is substantially accounted for by difference from the other oxides. The regression equation describing breakdown temperature vs. composition is as follows: T ^(breakdown)(predicted)(° C.)=2095.1−24.0[Al₂O₃]−8.5[B₂O₃]−33.6[Na₂O]−46.2[K₂O]−24.7[MgO]−23.9[CaO],  (4) where the concentrations in brackets are expressed in mol %. The coefficients in equation (4) have been further refined to yield the expression: T ^(breakdown)(predicted)(° C.)=2008.8−23.5[Al₂O₃]−1.6[B₂O₃]−33.6[Na₂O]−45.5[K₂O]−10.5[MgO+CaO].  (4a) The standard error of the regression in equations (4) and (4a) is about 7.3° C. The 2σ uncertainty is therefore about 15° C., which is close to the estimated uncertainty of the measurement itself.

Zircon breakdown temperature can also be experimentally determined by the isothermal hold technique. In this method, glass samples are placed in small platinum boats containing zircon refractory and held at a fixed temperature for a one week duration. While the isothermal hold method does not provide a unique temperature for zircon breakdown, it does serve as a screening tool. If it is known in advance that the breakdown temperature must be higher than a threshold value, then simply holding the glass sample and several other compositions simultaneously at the threshold temperature can be used to identify those samples having breakdown temperatures either above or below the threshold temperature.

Results of isothermal hold tests for glass compositions are listed in Table 2. Compositions listed in Table 2 are expressed in mol %. For nearly all of the samples listed, the compositions in Table 2 are nominal rather than measured values. The breakdown temperature of a sample cannot be uniquely determined via this technique. However, if zirconia is not observed, the breakdown temperature must be greater than the hold temperature. Likewise, if zirconia is observed, then the breakdown temperature must be lower than the isothermal hold temperature. The “sense” of the breakdown temperature relative to the hold temperature (i.e., greater or less than the hold temperature listed under “Breakdown T” in Table 2) is indicated in Table 2. The breakdown temperature calculated using the model previously described herein is also shown for comparison. For 75 samples listed in Table 2, the predicted breakdown temperature has the correct sense relative to the hold temperature; i.e., if the predicted temperature is greater than the hold temperature, then no zirconia is seen. Likewise, if the predicted temperature is less than the hold temperature, then varying amounts of zirconia is observed.

The difference between conventional glass having a low breakdown temperature and a glass having a high breakdown temperature, such as those disclosed herein is illustrated in FIGS. 3 and 4. FIG. 3 shows a backscattered electron micrograph of a glass composition (composition 5 in Table 1) having a low breakdown temperature, and FIG. 4 shows a backscattered electron micrograph of a glass compositions (composition 11 in Table 1) having a high zircon breakdown temperature. Both glasses were mixed with zircon grains and loaded into Pt boats, which were then placed in a furnace at 1175° C. for 114 hrs. The samples were then removed form the furnace, air quenched, and later cut and polished for inspection. As seen in FIG. 3, the sample containing glass composition 5, which has a breakdown temperature of 1105° C., clearly shows zirconia grains resulting from zircon breakdown. The zircon has reacted with the glass, as evidenced by the decoration of each of the zircon grains 310 (the grey phase in FIG. 3) with a large number of smaller zirconia grains 330 (white phase) just inside the glass 320 (dark phase). In contrast, the sample containing glass composition 11, which has a breakdown temperature of 1180° C., did not undergo zircon breakdown. FIG. 4 shows that zircon grains 310 and glass are present, but no zirconia grains were observed.

Relative amounts of zirconia observed for each sample are also listed in Table 2, and are described in relative amounts (“High,” Medium,” “Trace,” and “None”). The relative amounts of zirconia observed also track the breakdown temperature. If T^(breakdown) is predicted to be much lower than the hold temperature, a large amount of zirconia is observed. When the predicted T^(breakdown) is close to the isothermal hold temperature, only a trace of zirconia is observed. In the last ten samples listed in Table 2, the predicted sense of the breakdown temperature is not the same as observed, but all are within 2σ of the predicted breakdown temperature, and hence within the uncertainty of the prediction.

Two deficiencies are associated with the gradient strip and isothermal hold tests. First, the long exposures at high temperature that are used in both tests can produce volatilization of certain glass components—particularly B₂O₃—which can in turn impact the breakdown temperature of the glass. As discussed hereinabove, an increase in SiO₂ causes the breakdown temperature to increase. Since SiO₂ typically comprises more than 60 mol % of all the oxides present in the glasses described herein, the absolute SiO₂ concentration in the glass increases the most when a volatile component such as B₂O₃ is lost. Furthermore, at least one volatile component from a glass containing both Na₂O and B₂O₃ is Na₂B₄O₇, so both sodium and boron can be lost. Thus, the breakdown temperature for a glass having a high B₂O₃ concentration is likely to be an overestimate of the actual T^(breakdown) and the glass composition should be verified following the test. Second, the breakdown of zircon is very slow. Consequently, the one week hold time used in both types of tests may in some cases be insufficient to reveal an observable degree of breakdown. In general, both of these problems lead to observation of higher-than-expected breakdown temperatures.

Viscosity measurements were separately performed on a broad composition range of ion-exchangeable glasses. When the temperature is regressed to the temperature at which the viscosity of the glass is 35,000 poise (T^(35kp)), a linear correlation with composition is obtained: T ^(35kp)(° C.)=1484.5+17.1[Al₂O₃]−18.4[B₂O₃]−30.1[Na₂O]−26.0[K₂O]+0.9[MgO]−58.1[CaO],  (5) with a standard error of 5.8° C., and thus a propagated 2σ uncertainty of 11.6° C. Measured values of T^(35kp) are plotted against predicted 35 kp temperatures in FIG. 2, and show good correlation between values predicted in equation (5) and measured values. The coefficients in equation (5) have been further refined to yield the expression: T ^(35kp)(° C.)=1562.2+26.7[Al₂O₃]−24.2[B₂O₃]−38[Na₂O]−41.6[K₂O]−9.3[MgO+CaO].  (5a)

Since the breakdown temperature T^(breakdown) exceeds the 35 kp temperature T^(35kp), equations (1), (4), and (5) are combined to give equation (3): T ^(margin)=610.6−41.0[Al₂O₃]+9.9[B₂O₃]−3.5[Na₂O]−20.2[K₂O]−25.6[MgO]+34.2[CaO].  (3) The coefficients in equation (3) have been further refined to yield equation (3a): T ^(margin)(° C.)=446.6−50.2[Al₂O₃]+22.6[B₂O₃]−4.4[Na₂O]−3.9[K₂O]−1.2[MgO+CaO].  (3a) Exemplary glasses that satisfy equation (3) are listed in Table 3. Compositions listed in Table 3 are expressed in mol %.

In some embodiments, it is desirable that the zircon breakdown temperature is as high as possible, and preferably a temperature that is greater than any experienced on the isopipe. The propagated 2σ uncertainty is 19° C. To ensure that the breakdown temperature will be greater than any temperature in the isopipe, 19° C. can be subtracted from the regression coefficient in equations (3) and (3a) to account for any uncertainty: T ^(margin)=591.6−41.0[Al₂O₃]+9.9[B₂O₃]−3.5[Na₂O]−20.2[K₂O]−25.6[MgO]+34.2[CaO],  (6) T ^(margin)(° C.)=427.6−50.2[Al₂O₃]+22.6[B₂O₃]−4.4[Na₂O]−3.9[K₂O]−1.2[MgO+CaO].  (6a)

There is, however, sufficient flexibility in the fusion process to deliver glass at viscosities that are both lower and higher than 35 kp. Equation (3) could therefore be adopted, aiming for as high a breakdown margin T^(margin) as possible. The glass delivery temperature could then be adjusted if the required product attributes do not produce a predicted breakdown margin T^(margin) of greater than 19° C.

The glass compositions listed in Table 3 have a 350 poise temperature T³⁵⁰ (i.e., the temperature at which the glass is typically melted) that is less than or equal to 1650° C., which is desirable for reducing gaseous and solid inclusions at production scale melt rates. In addition, all of the compositions are ion exchangeable by those methods widely known in the art. In the ion exchange process, smaller metal ions in the glass are replaced or “exchanged” by larger metal ions of the same valence within a layer that is close to the outer surface of the glass. The replacement of smaller ions with larger ions creates a compressive stress within the layer. In one embodiment, the metal ions are monovalent alkali metal ions (e.g., Na⁺, K⁺, Rb⁺, and the like), and ion exchange is accomplished by immersing the glass in a bath comprising at least one molten salt of the larger metal ion that is to replace the smaller metal ion in the glass. Alternatively, other monovalent ions such as Ag⁺, Tl⁺, Cu⁺, and the like may be exchanged for monovalent ions. The ion exchange process or processes that are used to strengthen the glass can include, but are not limited to, immersion in a single bath or multiple baths of like or different compositions with washing and/or annealing steps between immersions.

In one embodiment, the glasses listed in Table 3 are ion exchanged by exposure to molten KNO₃ at a temperature of 410° C. for 8 hours to produce a compressive stress layer having a depth (also referred to as a “depth of layer”) of at least 20 μm on the surface of the glass and a maximum compressive stress of at least 350 MPa. In another embodiment the glasses listed in Table 3 are ion exchanged to achieve a central tension of at least 10 MPa.

TABLE 1 Gradient strip zircon breakdown temperatures. Predicted Breakdown breakdown Breakdown Sample SiO₂ Al₂O₃ B₂O₃ Na₂O K₂O MgO CaO SnO₂ T (° C.) T (° C.) margin (° C.) 1 61.94 13.99 5.99 13.99 2 2 0 0.1 1095 1096 −46 2 63.94 11.99 5.99 15.98 2 0 0 0.1 1115 1126 81 3 64.09 13.2 5.66 12.25 2.83 1.89 0 0.09 1140 1141 −24 4 63.94 11.99 5.99 13.99 2 2 0 0.1 1150 1144 37 5 66.12 10.19 0.58 14.16 2.37 5.75 0.59 0.21 1105 1103 −27 6 66.01 11.25 0.6 14.2 1.35 4.76 1.61 0.2 1120 1124 10 7 67.34 12.33 0 20.33 0 0 0 0 1125 1115 32 8 67.85 9.21 0.54 14.16 2.34 5.22 0.46 0.19 1140 1145 23 9 64.8 12.8 4.8 17.6 0 0 0 0 1150 1155 70 10 68.07 9.72 0.57 13.21 2.31 5.39 0.54 0.18 1160 1160 4 11 68.68 9.54 0.56 12.92 2.27 5.29 0.53 0.18 1180 1178 16 12 69.25 9.03 0.39 13.41 2.21 5.04 0.47 0.18 1185 1186 39 13 62.26 13.27 9.6 14.87 0 0 0 0 1195 1195 108 14 70.25 8.86 0 13.43 1.67 5.18 0.41 0.17 1210 1216 47 15 70.19 8.6 0 14.28 1.19 5.1 0.44 0.18 1215 1217 67 16 70.72 8.67 0 13 2.16 5.19 0.05 0.17 1220 1220 34 17 69.96 8.60 0.55 14.03 1.21 4.97 0.45 0.20 1215 1222 77 18 69.85 8.71 0.05 14.27 1.26 5.18 0.47 0.19 1200 1208 61 19 69.86 8.65 0.03 14.17 1.2 5.05 0.49 0.18 1220 1219 69 20 70.11 8.68 0.03 14.22 1.2 5.06 0.49 0.18 1215 1216 67 21 69.25 9.03 0.39 13.41 2.21 5.04 0.47 0.18 1190 1186 39 22 69.41 8.55 0 14.05 1.19 6.17 0.44 0.19 1215 1199 43 23 65.19 10.19 6.2 12.06 2.68 1.81 0.78 0.1 1215 1205 137 24 70.92 8.52 0 13.7 1.17 5.11 0.36 0.19 >1245 1241 70 25 62.97 8.91 2.23 15.46 4.84 5.34 0.08 0.15 <1090 985 −20 26 64.88 9.18 2.29 14.43 3.49 5.49 0.07 0.15 <1100 1071 −3 27 64.87 9.18 0.8 14.42 4.99 5.5 0.07 0.15 <1100 1015 −49 28 72.16 8.13 0 13.27 1.03 4.81 0.42 0.16 >1245 1277 100 29 73.51 7.6 0 11.58 2.3 4.2 0.6 0.2 >1250 1299 124 30 73.14 7.19 0 12.54 2.16 4.72 0.05 0.18 >1250 1283 108 31 73.52 7.6 0 13.87 0 4.2 0.61 0.2 >1260 1328 163 32 65.7 8 10 15.7 0.6 0 0 0.08 >1260 1262 314 33 65.91 10.3 6.27 12.19 2.71 1.83 0.79 0.1 1230 1195 132

TABLE 2 Results from isothermal zircon breakdown experiments. Predicted Zirconia Breakdown T breakdown Sample SiO₂ Al₂O₃ B2O₃ Na₂O K₂O MgO CaO SnO₂ amount (° C.) T (° C.) 34 65 11.25 0.6 14.25 2.35 5.75 0.6 0.2 High <1150 1076 35 64 12.25 0.6 14.25 2.35 5.75 0.6 0.2 High <1150 1052 36 64 11.25 0.6 14.25 2.35 5.75 1.6 0.2 High <1150 1052 37 62 12.25 0.6 14.25 2.35 5.75 2.6 0.2 High <1150 1004 6 66 11.25 0.6 14.25 1.35 4.75 1.6 0.2 Trace <1150 1123 38 66 12.25 0.6 14.25 0.35 3.75 2.6 0.2 Medium <1150 1146 39 66 11.3 0.6 14.2 1.4 5.2 1.1 0.2 Medium <1150 1122 40 66 11.3 0.6 14.2 1.4 5.7 0.6 0.2 Medium <1150 1121 41 65.35 11.19 1.58 14.06 1.34 4.7 1.58 0.2 Medium <1150 1124 42 65.34 11.19 1.58 14.06 1.39 5.15 1.09 0.2 Medium <1150 1122 29 73.5 7.6 0 11.6 2.3 4.2 0.6 0.2 None >1190 1298 43 72.55 8.2 0 11.8 2.3 4.35 0.6 0.2 None >1190 1273 44 71.6 8.85 0 12 2.3 4.45 0.6 0.2 None >1190 1248 45 70.7 9.45 0 12.2 2.3 4.55 0.6 0.2 None >1190 1225 46 69.75 10.05 0 12.4 2.3 4.7 0.6 0.2 None >1190 1200 47 69.87 8.77 0.56 12.67 2.27 5.14 0.53 0.18 None >1170 1209 48 71.04 8 0.56 12.42 2.27 4.99 0.53 0.18 None >1170 1240 49 72.22 7.22 0.56 12.17 2.27 4.84 0.53 0.18 None >1170 1270 50 5170.43 8.77 0 12.67 2.27 5.14 0.53 0.18 None >1170 1214 51 71.61 7.99 0 12.42 2.27 4.99 0.53 0.18 None >1170 1245 52 72.78 7.22 0 12.17 2.27 4.84 0.53 0.18 None >1170 1275 16 70.72 8.67 13 2.16 5.19 0.05 0.17 None >1190 1220 53 71.61 7.99 0 12.95 2.27 4.99 0 0.18 None >1190 1239 30 73.14 7.19 12.54 2.16 4.72 0.05 0.18 None >1190 1283 54 72.22 7.22 0 12.17 3.36 4.84 0 0.18 None >1190 1238 55 72.22 7.22 0 14.26 2.27 3.84 0 0.18 None >1190 1242 56 72.22 7.22 0 13.26 3.27 3.84 0 0.18 None >1190 1230 57 73.5 7.6 0 12.75 1.15 4.2 0.6 0.2 None >1200 1312 58 73.5 7.6 0 13.9 0 4.2 0.6 0.2 None >1200 1327 59 72.55 8.2 0 12.95 1.15 4.35 0.6 0.2 None >1200 1288 60 72.55 8.2 0 14.1 0 4.35 0.6 0.2 None >1200 1302 61 73.94 7.65 0 13.98 0 4.23 0 0.2 None >1200 1337 62 72.98 8.25 0 14.19 0 4.38 0 0.2 None >1200 1311 63 73.5 7.6 0 11.6 2.3 4.2 0.6 0.2 None >1200 1298 31 73.5 7.6 0 13.9 0 4.2 0.6 0.2 None >1200 1327 64 62.93 12.99 5 16.98 2 0 0 0.1 Medium <1100 1077 65 61.94 13.99 5.99 15.98 2 0 0 0.1 High <1150 1078 66 61.93 13.99 5.99 13.99 4 0 0 0.1 High <1150 1053 67 61.93 13.99 5.99 13.99 2 2 0 0.1 Medium <1150 1096 68 61.93 13.99 5 16.98 2 0 0 0.1 High <1150 1053 69 61.93 13.99 4 17.98 2 0 0 0.1 High <1150 1028 8 67.87 9.21 0.54 14.16 2.34 5.22 0.46 0.19 Trace <1170 1145 70 67.732 9.696 0.6185 13.336 2.3738 5.5414 0.4871 0.1855 Trace <1170 1150 71 68.67 8.54 0.56 13.96 2.27 5.29 0.53 0.18 Trace <1190 1167 72 71.15 7.78 0.51 12.93 2.09 4.87 0.49 0.18 None >1190 1240 73 71.16 8.03 0 13.18 2.09 4.87 0.49 0.18 None >1190 1230 74 63.94 12.99 8.99 13.99 0 0 0 0.1 None >1200 1236 75 63.94 13.49 8.99 13.49 0 0 0 0.1 None >1200 1241 76 63.94 13.99 8.99 12.99 0 0 0 0.1 None >1200 1246 77 63.94 14.49 7.99 13.49 0 0 0 0.1 None >1200 1225 78 63.94 12.49 9.99 13.49 0 0 0 0.1 None >1200 1256 79 63.94 13.49 8.99 11.49 2 0 0 0.1 None >1200 1216 80 63.94 14.49 6.99 14.49 0 0 0 0.1 None >1200 1200 81 63.94 12.49 10.99 12.49 0 0 0 0.1 None >1200 1281 82 63.94 11.99 11.99 11.99 0 0 0 0.1 None >1200 1302 83 63.94 13.49 7.99 13.49 0 1 0 0.1 None >1200 1225 84 63.94 13.49 6.99 13.49 0 2 0 0.1 None >1200 1208 85 68.67 9.68 0 13.37 2.27 5.29 0.53 0.18 None >1170 1165 86 68.67 9.54 0 13.51 2.27 5.29 0.53 0.18 Trace <1200 1163 15 70.194 8.6009 0 14.283 1.1848 5.098 0.4377 0.1751 None >1200 1217 14 70.223 8.8623 0 13.422 1.6727 5.1828 0.4383 0.1716 None >1200 1215 87 71.27 8.24 0 12.9 2.08 4.85 0.48 0.17 None >1200 1236 88 67.2 12.31 0 20.29 0 0 0 0.2 Medium <1150 1117 89 67.01 12.35 0.36 20.08 0 0 0 0.2 High <1150 1120 90 66.82 12.38 0.72 19.88 0 0 0 0.2 High <1150 1123 91 68.32 11.82 0.69 18.97 0 0 0 0.2 None >1150 1167 92 67.77 10.84 0.36 20.83 0 0 0 0.2 Trace <1150 1131 93 67.57 10.88 0.72 20.63 0 0 0 0.2 Trace <1150 1134 94 67.2 11.31 0 19.29 0 2 0 0.2 Medium <1150 1125 95 67.02 11.34 0.36 19.08 0 2 0 0.2 Medium <1150 1129 96 66.82 11.38 0.72 18.88 0 2 0 0.2 Trace <1150 1131 97 68.7 10.74 0 18.36 0 2 0 0.2 None >1150 1170 98 68.52 10.78 0.34 18.16 0 2 0 0.2 None >1150 1173 99 68.31 10.82 0.69 17.98 0 2 0 0.2 None >1150 1175 100 66.14 10.19 0.58 14.16 2.37 5.75 0.59 0.21 Medium <1150 1103 101 66 12.3 0.6 14.2 0.35 4.75 1.6 0.2 None >1150 1145 102 61.88 13.97 5.99 17.96 0 0 0 0.2 Trace <1100 1105 103 68.67 10.04 0.56 12.46 2.27 5.29 0.53 0.18 None >1190 1182 104 63.94 13.49 5.99 13.49 0 3 0 0.1 None >1200 1192 105 69.06 9.59 0 13.03 2.28 5.32 0.53 0.18 None >1190 1177 106 68.7 11.74 0 19.36 0 0 0 0.2 Trace <1150 1162 107 68.52 11.78 0.34 19.16 0 0 0 0.2 Trace <1150 1165 108 67.96 13.05 0 18.79 0 0 0 0.2 Trace <1150 1150 109 67.78 13.08 0.36 18.58 0 0 0 0.2 Medium <1150 1153 110 67.58 13.12 0.72 18.38 0 0 0 0.2 Trace <1150 1156 111 69.57 8.43 0 13.95 1.16 6.18 0.5 0.18 None >1200 1206 112 69.9 8.3 0 13.81 1.15 6.14 0.49 0.18 None >1200 1215 113 69.4 8.36 0 14.29 1.06 6.18 0.5 0.18 None >1200 1201 114 69.95 8.23 0 14.01 1.04 6.07 0.5 0.18 None >1200 1217

TABLE 3 Glass compositions that satisfy equation (3). Predicted Breakdown breakdown 35 kp margin Sample SiO₂ Al₂O₃ B₂O₃ Na₂O K₂O MgO CaO SnO₂ T (° C.) T (° C.) prediction predict 6 66.01 11.25 0.6 14.2 1.35 4.76 1.61 0.2 1120 1124 1113 11 38 66 12.25 0.6 14.25 0.35 3.75 2.6 0.2 <1150 1146 1096 50 41 65.35 11.19 1.58 14.06 1.34 4.7 1.58 0.2 <1150 1124 1100 24 29 73.51 7.6 0 11.58 2.3 4.2 0.6 0.2 >1250 1299 1175 124 43 72.55 8.2 0 11.8 2.3 4.35 0.6 0.2 >1190 1273 1178 95 44 71.6 8.85 0 12 2.3 4.45 0.6 0.2 >1190 1248 1184 64 45 70.7 9.45 0 12.2 2.3 4.55 0.6 0.2 >1190 1225 1188 37 46 69.75 10.05 0 12.4 2.3 4.7 0.6 0.2 >1190 1200 1192 8 47 69.87 8.77 0.56 12.67 2.27 5.14 0.53 0.18 >1170 1209 1157 52 48 71.04 8 0.56 12.42 2.27 4.99 0.53 0.18 >1170 1240 1151 89 49 72.22 7.22 0.56 12.17 2.27 4.84 0.53 0.18 >1170 1270 1145 125 50 70.43 8.77 0 12.67 2.27 5.14 0.53 0.18 >1170 1214 1167 47 51 71.61 7.99 0 12.42 2.27 4.99 0.53 0.18 >1170 1245 1162 83 52 72.78 7.22 0 12.17 2.27 4.84 0.53 0.18 >1170 1275 1156 119 16 70.72 8.67 13 2.16 5.19 0.05 0.17 >1190 1220 1187 33 16 70.72 8.67 0 13 2.16 5.19 0.05 0.17 1220 1220 1187 33 53 71.61 7.99 0 12.95 2.27 4.99 0 0.18 >1190 1239 1176 63 30 73.14 7.19 0 12.54 2.16 4.72 0.05 0.18 >1250 1283 1175 108 54 72.22 7.22 0 12.17 3.36 4.84 0 0.18 >1190 1238 1158 80 55 72.22 7.22 0 14.26 2.27 3.84 0 0.18 >1190 1242 1123 119 56 72.22 7.22 0 13.26 3.27 3.84 0 0.18 >1190 1230 1127 103 57 73.5 7.6 0 12.75 1.15 4.2 0.6 0.2 >1200 1312 1169 143 58 73.5 7.6 0 13.9 0 4.2 0.6 0.2 >1200 1327 1164 163 59 72.55 8.2 0 12.95 1.15 4.35 0.6 0.2 >1200 1288 1174 114 60 72.55 8.2 0 14.1 0 4.35 0.6 0.2 >1200 1302 1169 133 61 73.94 7.65 0 13.98 0 4.23 0 0.2 >1200 1337 1198 139 62 72.98 8.25 0 14.19 0 4.38 0 0.2 >1200 1311 1202 109 63 73.5 7.6 0 11.6 2.3 4.2 0.6 0.2 >1200 1298 1174 124 31 73.52 7.6 0 13.87 0 4.2 0.61 0.2 >1260 1328 1165 163 102 61.88 13.97 5.99 17.96 0 0 0 0.2 <1100 1105 1072 33 2 63.94 11.99 5.99 15.98 2 0 0 0.1 1115 1126 1046 80 4 63.94 11.99 5.99 13.99 2 2 0 0.1 1150 1144 1107 37 64 62.93 12.99 5 16.98 2 0 0 0.1 <1100 1077 1051 26 10 68.07 9.72 0.57 13.21 2.31 5.39 0.54 0.18 1160 1160 1155 5 11 68.68 9.54 0.56 12.92 2.27 5.29 0.53 0.18 1180 1178 1163 15 8 67.85 9.21 0.54 14.16 2.34 5.22 0.46 0.19 1140 1145 1122 23 71 68.67 8.54 0.56 13.96 2.27 5.29 0.53 0.18 <1190 1167 1114 53 72 71.15 7.78 0.51 12.93 2.09 4.87 0.49 0.18 >1190 1240 1140 100 73 71.16 8.03 0 13.18 2.09 4.87 0.49 0.18 >1190 1230 1146 84 74 63.94 12.99 8.99 13.99 0 0 0 0.1 >1200 1236 1119 117 75 63.94 13.49 8.99 13.49 0 0 0 0.1 >1200 1241 1143 98 76 63.94 13.99 8.99 12.99 0 0 0 0.1 >1200 1246 1166 80 77 63.94 14.49 7.99 13.49 0 0 0 0.1 >1200 1225 1178 47 78 63.94 12.49 9.99 13.49 0 0 0 0.1 >1200 1256 1107 149 79 63.94 13.49 8.99 11.49 2 0 0 0.1 >1200 1216 1151 65 80 63.94 14.49 6.99 14.49 0 0 0 0.1 >1200 1200 1167 33 81 63.94 12.49 10.99 12.49 0 0 0 0.1 >1200 1281 1119 162 82 63.94 11.99 11.99 11.99 0 0 0 0.1 >1200 1302 1107 195 83 63.94 13.49 7.99 13.49 0 1 0 0.1 >1200 1225 1162 63 84 63.94 13.49 6.99 13.49 0 2 0 0.1 >1200 1208 1181 27 85 68.67 9.68 0 13.37 2.27 5.29 0.53 0.18 >1170 1165 1162 3 105 69.06 9.59 0 13.03 2.28 5.32 0.53 0.18 >1190 1177 1170 7 32 65.7 8 10 15.7 0.6 0 0 0.08 >1260 1262 948 314 86 68.67 9.54 0 13.51 2.27 5.29 0.53 0.18 <1200 1163 1155 8 15 70.19 8.6 0 14.28 1.19 5.1 0.44 0.18 1215 1217 1149 68 14 70.25 8.86 0 13.43 1.67 5.18 0.41 0.17 1210 1216 1169 47 87 71.27 8.24 0 12.9 2.08 4.85 0.48 0.17 >1200 1236 1159 77 28 72.16 8.13 0 13.27 1.03 4.81 0.42 0.16 >1245 1277 1177 100 22 69.41 8.55 0 14.05 1.19 6.17 0.44 0.19 1215 1199 1156 43 24 70.92 8.52 0 13.7 1.17 5.11 0.36 0.19 >1260 1241 1171 70 20 70.11 8.68 0.03 14.22 1.2 5.06 0.49 0.18 1215 1216 1149 67 23 65.19 10.19 6.2 12.06 2.68 1.81 0.78 0.1 1215 1205 1068 137 12 69.25 9.03 0.39 13.41 2.21 5.04 0.47 0.18 1185 1186 1147 39 17 69.96 8.60 0.55 14.03 1.21 4.97 0.45 0.20 1215 1222 1145 77 18 69.85 8.71 0.05 14.27 1.26 5.18 0.47 0.19 1200 1208 1147 61 19 69.86 8.65 0.03 14.17 1.2 5.05 0.49 0.18 1220 1219 1150 69 33 65.91 10.3 6.27 12.19 2.71 1.83 0.79 0.1 1230 1195 1063 132 13 62.26 13.27 9.6 14.87 0 0 0 0 1195 1195 1086 109 9 64.8 12.8 4.8 17.6 0 0 0 0 1150 1155 1085 70 7 67.34 12.33 0 20.33 0 0 0 0 1125 1115 1083 32 88 67.2 12.31 0 20.29 0 0 0 0.2 <1150 1117 1084 33 89 67.01 12.35 0.36 20.08 0 0 0 0.2 <1150 1120 1084 36 90 66.82 12.38 0.72 19.88 0 0 0 0.2 <1150 1123 1084 39 106 68.7 11.74 0 19.36 0 0 0 0.2 <1150 1162 1102 60 107 68.52 11.78 0.34 19.16 0 0 0 0.2 <1150 1165 1102 63 91 68.32 11.82 0.69 18.97 0 0 0 0.2 >1150 1167 1102 65 108 67.96 13.05 0 18.79 0 0 0 0.2 <1150 1150 1141 9 109 67.78 13.08 0.36 18.58 0 0 0 0.2 <1150 1153 1142 11 110 67.58 13.12 0.72 18.38 0 0 0 0.2 <1150 1156 1142 14 92 67.77 10.84 0.36 20.83 0 0 0 0.2 <1150 1131 1036 95 93 67.57 10.88 0.72 20.63 0 0 0 0.2 <1150 1134 1036 98 94 67.2 11.31 0 19.29 0 2 0 0.2 <1150 1125 1098 27 95 67.02 11.34 0.36 19.08 0 2 0 0.2 <1150 1129 1099 30 96 66.82 11.38 0.72 18.88 0 2 0 0.2 <1150 1131 1099 32 97 68.7 10.74 0 18.36 0 2 0 0.2 >1150 1170 1117 53 98 68.52 10.78 0.34 18.16 0 2 0 0.2 >1150 1173 1117 56 99 68.31 10.82 0.69 17.98 0 2 0 0.2 >1150 1175 1117 58 111 69.57 8.43 0 13.95 1.16 6.18 0.5 0.18 >1200 1206 1155 52 112 69.9 8.3 0 13.81 1.15 6.14 0.49 0.18 >1200 1215 1158 58 113 69.4 8.36 0 14.29 1.06 6.18 0.5 0.18 >1200 1201 1146 55 114 69.95 8.23 0 14.01 1.04 6.07 0.5 0.18 >1200 1217 1153 65

While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the disclosure or appended claims. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present disclosure or appended claims. 

The invention claimed is:
 1. A glass comprising SiO₂, Al₂O₃, and Na₂O, wherein SiO₂+B₂O₃≧66 mol %, Al₂O₃≧7 mol %, B₂O₃≦6.27 mol %, Na₂O≧9 mol %, B₂O₃+Na₂O+K₂O+MgO+CaO≧18 mol %, and CaO≦2 mol %, wherein the glass has a temperature T^(35kp) at which the glass has a viscosity of 35 kpoise and a zircon breakdown temperature T^(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂, wherein T^(breakdown) is greater than T^(35kp) wherein T^(margin)=T^(breakdown)−T^(35kp), wherein T^(margin) (° C.)=446.6−50.2[Al₂O₃]+22.6[B₂O₃]−4.4[Na₂O]−3.9[K₂O]−1.2[MgO+CaO] where concentrations [Al₂O₃], [B₂O₃], [Na₂O], [K₂O], [MgO], and [CaO] are expressed in mol %, and wherein T^(margin)≧0.
 2. The glass of claim 1, wherein the glass comprises: 61 mol %≦SiO₂≦75 mol %; 7 mol %≦Al₂O₃≦15 mol %; 0 mol %≦B₂O₃≦6.27 mol %; 9 mol %≦Na₂O≦21 mol %; 0 mol %≦K₂O≦4 mol %; 0 mol %≦MgO≦7 mol %; and 0 mol %≦CaO≦2 mol %.
 3. The glass of claim 1, wherein T^(margin)=T^(breakdown)−T^(35kp), wherein T^(margin)=610.6−41.0[Al₂O₃]+9.9[B₂O₃]−3.5[Na₂O]−20.2[K₂O]−25.6[MgO]+34.2[CaO] where concentrations [Al₂O₃], [B₂O₃], [Na₂O], [K₂O], [MgO], and [CaO] are expressed in mol %, and wherein T^(margin)≧0.
 4. The glass of claim 1, wherein the glass is down-drawable.
 5. The glass of claim 1, wherein the glass is fusion drawn into a glass sheet.
 6. The glass of claim 5, wherein the glass sheet has less than one inclusion of solid ZrO₂ per pound of glass sheet.
 7. The glass of claim 1, wherein B₂O₃+Na₂O+K₂O−Al₂O₃≧0 mol %.
 8. The glass of claim 1, wherein the glass has a liquidus viscosity of greater than about 100 kpoise.
 9. The glass of claim 1, wherein the glass has a temperature at a viscosity of 350 poise, T³⁵⁰, of less than or equal to 1650° C.
 10. The glass of claim 1, wherein the glass is formed into one of a cover plate, a window, a casing, a display screen, and a touch panel of an electronic device.
 11. The glass of claim 1, wherein the glass is ion exchangeable.
 12. The glass of claim 11, wherein the glass is ion exchanged to form a compressive layer on at least one surface of the glass.
 13. The glass of claim 12, wherein the ion exchanged glass has a compressive stress of at least 350 MPa and a compressive depth of layer of at least 20 microns.
 14. The glass of claim 12, wherein the ion exchanged glass has a central tension of at least 10 MPa.
 15. A glass, the glass comprising SiO₂, Al₂O₃, and Na₂O and having a temperature T^(35kp) at which the glass has a viscosity of 35 kpoise, wherein SiO₂+B₂O₃≧66 mol %, Al₂O₃≧7 mol %, B₂O₃≦6.27 mol %, Na₂O≧9 mol %, B₂O₃+Na₂O+K₂O+MgO+CaO≧18 mol %, and CaO≦2 mol %, and having a temperature T^(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂, wherein T^(breakdown) is greater than T^(35kp), wherein T^(margin)=T^(breakdown)−T^(35kp), wherein T^(margin) (° C.)=446.6−50.2[Al₂O₃]+22.6[B₂O₃]−4.4[Na₂O]−3.9[K₂O]−1.2[MgO+CaO] where concentrations [Al₂O₃], [B₂O₃], [Na₂O], [K₂O], [MgO], and [CaO] are expressed in mol %, and wherein T^(margin)≧0.
 16. The glass of claim 15, wherein the glass comprises: 61 mol %≦SiO₂≦75 mol %; 7 mol %≦Al₂O₃≦15 mol %; 0 mol %≦B₂O₃≦6.27 mol %; 9 mol %≦Na₂O≦21 mol %; 0 mol %≦K₂O≦4 mol %; 0 mol %≦MgO≦7 mol %; and 0 mol %≦CaO≦2 mol %.
 17. The glass of claim 15, wherein T^(margin)=T^(breakdown)−T^(35kp), wherein T^(margin)=610.6−41.0[Al₂O₃]+9.9[B₂O₃]−3.5[Na₂O]−20.2[K₂O]−25.6[MgO]+34.2[CaO] where concentrations [Al₂O₃], [B₂O₃], [Na₂O], [K₂O], [MgO], and [CaO] are expressed in mol %, and wherein T^(margin)≧0.
 18. The glass of claim 15, wherein the glass is down-drawable.
 19. The glass of claim 15, wherein the glass is fusion drawn into a glass sheet.
 20. The glass of claim 19, wherein the glass sheet has less than one inclusion of solid ZrO₂ per pound of glass sheet.
 21. The glass of claim 15, wherein B₂O₃+Na₂O+K₂O−Al₂O₃≧0 mol %.
 22. The glass of claim 15, wherein the glass has a liquidus viscosity of greater than about 100 kpoise.
 23. The glass of claim 15, wherein the glass has a temperature at a viscosity of 350 poise, T³⁵⁰, of less than or equal to 1650° C.
 24. The glass of claim 15, wherein the glass is formed into one of a cover plate, a window, a casing, a display screen, and a touch panel of an electronic device.
 25. The glass of claim 15, wherein the glass is ion exchangeable.
 26. The glass of claim 19, wherein the glass is ion exchanged to form a compressive layer on at least one surface of the glass.
 27. The glass of claim 26, wherein the ion exchanged glass has a compressive stress of at least 350 MPa and a compressive depth of layer of at least 20 microns.
 28. The glass of claim 26, wherein the ion exchanged glass has a central tension of at least 10 MPa.
 29. A glass comprising SiO₂, Al₂O₃, and Na₂O, wherein SiO₂+B₂O₃≧66 mol %, Al₂O₃≧7 mol %, B₂O₃≦6.27 mol %, Na₂O≧9 mol %, B₂O₃+Na₂O+K₂O+MgO+CaO≧18 mol %, and CaO≦2 mol %, and having a breakdown margin T^(margin), wherein T^(margin) (° C.)=446.6−50.2[Al₂O₃]+22.6[B₂O₃]−4.4[Na₂O]−3.9[K₂O]−1.2[MgO+CaO], where concentrations [Al₂O₃], [B₂O₃], [Na₂O], [K₂O], [MgO], and [CaO] are expressed in mol %, and wherein T^(margin)≧0.
 30. A glass comprising SiO₂, Al₂O₃, and at least one alkali metal oxide which includes Na₂O and, optionally, at least one of K₂O and Rb₂O, wherein SiO₂+B₂O₃≧66 mol %, Al₂O₃≧7 mol %, B₂O₃≦6.27 mol %, Na₂O≧9 mol %, B₂O₃+Na₂O+K₂O+MgO+CaO≧18 mol %, and CaO≦2 mol %, and having a breakdown margin T^(margin), wherein T^(margin)=T^(breakdown)−T^(35kp), wherein T^(margin) (° C.)=446.6−50.2[Al₂O₃]+22.6[B₂O₃]−4.4[Na₂O]−3.9[K₂O]−1.2[MgO+CaO] where concentrations [Al₂O₃], [B₂O₃], [Na₂O], [K₂O], [MgO], and [CaO] are expressed in mol %, and wherein T^(margin)≧0.
 31. The glass of claim 30, wherein the glass has a temperature T^(35kp) at which the glass has a viscosity of 35 kpoise and a zircon breakdown temperature T^(breakdown) at which zircon breaks down to form ZrO₂ and SiO₂, wherein T^(breakdown) is greater than T^(35kp), and wherein T^(margin)=T^(breakdown)−T^(35kp).
 32. The glass of claim 30, wherein the glass comprises: 61 mol %≦SiO₂≦75 mol %; 7 mol %≦Al₂O₃≦15 mol %; 0 mol %≦B₂O₃≦6.27 mol %; 9 mol %≦Na₂O≦21 mol %; 0 mol %≦K₂O≦4 mol %; 0 mol %≦MgO≦7 mol %; and 0 mol %≦CaO≦2 mol %.
 33. The glass of claim 30, wherein the glass has less than one inclusion of solid ZrO₂ per pound of glass.
 34. The glass of claim 30, wherein the glass is ion exchanged to form a compressive layer on at least one surface of the glass, and wherein the ion exchanged glass has a compressive stress of at least 350 MPa and a compressive depth of layer of at least 20 microns.
 35. The glass of claim 34, wherein the ion exchanged glass has a central tension of at least 10 MPa. 